Usually a shower door needs a horizontal track and a vertical side frame so as to be openably and closably installed to a shower room, whereby the shower door, the horizontal track and the vertical side frame form a shower door assembly. When installing a shower door, the horizontal track, the vertical side frame and the shower door need to be assembled on site. According to an existing installation method, holes are provided to the vertical side frame; then, the horizontal track, the vertical side frame and the shower door are placed in the installation position of the shower door; the horizontal track and the vertical side frame are mounted together using screws. Due to restrictions of the installation position, such an installation method is time-consuming, and screws often slip or facture, which requires much labor from installation persons.